Speed ratio controller and control method of continuously variable transmission

Abstract

In a vehicle continuously variable transmission, a controller (61) determines the transient target speed ratio Ratio0. A real speed ratio Ratio is detected by sensors (64, 65). The controller (61) calculates a feedback correction amount FBrto based on a deviation RtoERR of the real speed ratio Ratio from the transient target speed ratio Ratio0. The controller (61) then calculates a command value DsrSTP from the transient target speed ratio Ratio0 and feedback correction amount FBrto. The controller (61) applies a specific limitation to the command value DsrSTP when the real speed ratio Ratio is out of a predetermined range, so as to prevent the real speed ratio Ratio from exceeding the mechanical operation limit of the transmission. The limitation is based on the command value DsrSTP calculated in the preceding control cycle.

What is claimed is: 
     
       1. A speed ratio controller for use with a continuously variable transmission for a vehicle, said controller controlling a speed ratio of said transmission in a predetermined control cycle, said controller comprising: a sensor for detecting a running condition of the vehicle,   a sensor for detecting a real speed ratio of the transmission, and   a microprocessor programmed to: calculate a transient target speed ratio of the transmission based on said vehicle running condition,   calculate a deviation between said real speed ratio and said transient target speed ratio,   calculate a feedback correction amount based on said deviation,   calculate a command value based on said transient target speed ratio and said feedback correction amount,   determine if said real speed ratio is in a predetermined range,   apply a specific limitation to said command value when said real speed ratio is out of said predetermined range, and   control the speed ratio of said transmission based on a limited command value.     
     
     
       2. A speed ratio controller as defined in claim 1, wherein said predetermined range is set according to a mechanical operation limit of the transmission, and said microprocessor is further programmed to prevent said command value, when said real speed ratio is larger than said predetermined range, from becoming larger than the command value calculated in a preceding control cycle, and prevent said command value, when said real speed ratio is smaller than said predetermined range, from becoming smaller than the command value. 
     
     
       3. A speed ratio controller as defined in claim 2, wherein said microprocessor is further programmed to calculate a difference of said transient target speed ratio and the command value calculated in the preceding control cycle as a feedback correction limiting value, and limit said feedback correction amount by said feedback correction limiting value. 
     
     
       4. A speed ratio controller as defined in claim 3, wherein said feedback correction amount comprises an integral correction amount obtained by applying a predetermined integral processing to said deviation, and said microprocessor is further programmed to limit said integral correction amount by said feedback correction limiting value. 
     
     
       5. A speed ratio controller as defined in claim 1, wherein said transmission is connected to an engine of which the output varies according to the opening of a throttle, and wherein said running condition detecting sensor comprises a sensor for detecting said throttle opening, and a sensor for detecting a vehicle speed. 
     
     
       6. A speed ratio controller for use with a continuously variable transmission for a vehicle, said controller controlling a speed ratio of said transmission in a predetermined control cycle, said controller comprising: means for detecting a running condition of the vehicle,   means for detecting a real speed ratio of the transmission,   means for calculating a transient target speed ratio of the transmission based on said vehicle running condition,   means for calculating a deviation between said real speed ratio and said transient target speed ratio,   means for calculating a feedback correction amount based on said deviation,   means for calculating a command value based on said transient target speed ratio and said feedback correction amount,   means for determining if said real speed ratio is in a predetermined range,   means for applying a specific limitation to said command value when said real speed ratio is out of said predetermined range, and   means for controlling the speed ratio of said transmission based on a limited command value.   
     
     
       7. A speed ratio control method for controlling a speed ratio of a continuously variable transmission for a vehicle in a predetermined control cycle, said method comprising: detecting a running condition of the vehicle,   detecting a real speed ratio of the transmission,   calculating a transient target speed ratio of the transmission based on said vehicle running condition,   calculating a deviation between said real speed ratio and said transient target speed ratio,   calculating a feedback correction amount based on said deviation,   calculating a command value based on said transient target speed ratio and said feedback correction amount,   determining if said real speed ratio is in a predetermined range,   applying a specific limitation to said command value when said real speed ratio is out of said predetermined range, and   controlling the speed ratio of said transmission based on a limited command value.